Add const specifier for not changed strings.

[tobbad]

I try to integrate uGui into micropython when I saw these small issues. In my opinion a string which is not modified in the receiving function should be declared as const. This is as well needed by the function which call the ugui function where I receive a const char *. Further there are some small fixes which satisfy a picky gcc. I have a little bit problem with this latin-1 encoded file in eclipse and some fixes below show, that others had it, too. Why not just use the regular ASCII set?

[jmoyerman]

Good stuff.

Const modifiers are always useful wherever you can put them.

I've messaged Achim before, he hasnt abandoned the project, just University. Ill merge this into my branch when I get infront of a PC.